Field of the Invention
The invention relates to an instrument for assembling a polyaxial bone anchor. A polyaxial bone anchor typically includes a bone anchoring element and a receiving part that is configured to pivotably receive the bone anchoring element. The angular position of the bone anchoring element relative to the receiving part can be locked by exerting pressure onto a head of the bone anchoring element. The instrument includes a first member configured to engage the receiving part and a second member configured to contact the bone anchoring element and a position indicator that indicates the position of the bone anchoring element relative to the receiving part. The instrument is particularly suitable for the in situ assembly of a bottom loading polyaxial bone anchor.
Description of Related Art
A tool for assembling a polyaxial bone anchor is known, for example, from US 2012/0124813 A1. The polyaxial bone anchor is a bottom loading polyaxial bone anchor that comprises an outer locking ring configured to be positioned around a head receiving portion of the receiving part and to exert pressure onto the head of the bone anchoring element. With the tool, the bone anchoring element and the head receiving part with the locking ring can be assembled before the bone anchor is inserted into a bone part or a vertebra.
Other bottom loading polyaxial bone anchors are known that comprise an inner compression member to exert pressure onto the head of the bone anchoring element. Such bottom loading polyaxial bone anchors that allow for in situ assembly, i.e. inserting the bone anchoring element into the bone first and thereafter assembling the receiving part with the head of the bone anchoring element, are known, for example, from U.S. Pat. No. 6,063,090 or from U.S. Pat. No. 8,034,089 B2. In the case of in situ mounting of the receiving part onto the head of the anchoring element, in certain situations it might be difficult to recognize whether the receiving part has been correctly mounted onto the bone anchoring element.